This invention relates to amorphous metal alloys and particularly to amorphous metal alloy coatings for protecting metals against corrosion.
Metallic glasses are relatively new materials that are produced in the form of ribbons, wires, or coatings by rapid solidification from either the liquid or vapor state. They are truly noncrystalline alloys which combine the properties of a metal (electrical conductivity, metallic luster, etc.) with the lack of long range order typical of a glass. They are very homogeneous and lack the defects typical of a crystalline material, such as grain boundaries and dislocations.
There are a large number of alloy systems which will form metallic glasses. The best known metallic glasses combine late transition metals with metalloids such as boron, carbon, silicon, and phosphorus. Because of the lack of stoichiometric requirements, the composition of a metallic glass can be varied continuously within certain limits. Thus, the properties of a metallic glass can be tailored over a wide range to optimize various properties.
The outstanding corrosion resistance of some metallic glasses derives from two sources. The complete lack of microstructure leads to a particularly homogeneous passive film with no underlying defects to serve as pitting sites. In addition, the surface of an amorphous solid has a higher free energy than the surface of the corresponding crystalline solid leading to more aggressive passivation. It should be noted that if the metallic glass lacks elements that form a passive film, such as chromium or aluminum, the higher reactivity of the surface results in a corrosion resistance which is poorer than that of the corresponding crystalline material. If, however, the appropriate element is present, metallic glasses produce a higher quality protective film than that of a crystalline material with similar composition.
The outstanding corrosion properties of metallic glasses have led to several research efforts aimed at producing corrosion resistant coatings on metallic substrates. These include Mo-Ru-B, USN S30400 stainless steel (304SS) with small additions of carbon, Mo-Cr-B, and a sputtered version of Metglas.RTM. 2826A. Procedures for fabricating highly corrosion-resistant metallic glass coatings are now very well established. The method of choice is usually magnetron sputtering, an industrially mature process which is characterized by high deposition rate and excellent film adhesion. Recently, amorphous metal coatings also have been produced by electroless deposition.
There are a number of situations in which a metallic glass coating would have a distinct advantage over other types of corrosion protection. Amorphous metal coatings would be especially useful in cases where the coating must be electrically conductive (e.g., electrical contacts) or where the dimension of the work-piece must remain unchanged within a certain tolerance. Another major advantage of metallic glasses is that because they have no grain boundaries or defects, they tend to make excellent barriers to permeation of various corrosive substances such as fuel components. Such permeation can cause severe corrosion problems.
Unfortunately, a small pinhole or scratch often results in the severe corrosion and catastrophic failure of the underlying crystalline metal structure material. It would therefore be desirable to provide corrosion-resistant coatings that could have pinholes, cracks, or scratches and still protect the underlying crystalline metal material from major corrosion.